The present invention relates to voltage-controlled oscillators. More particularly, the present invention relates to a fuse-trimmed tank circuit for an integrated voltage-controlled oscillator.
A voltage-controlled oscillator (VCO) is an electronic device which provides a periodic signal where the frequency of the periodic signal is related to the level of an input voltage control signal supplied to the VCO. There are numerous applications for VCOs, including modulating and demodulating radio frequency communication signals, such as in a radio frequency transceiver. A VCO typically includes a resonant tank circuit which resonates so as to form the periodic signal. A conventional resonant tank circuit used in an integrated VCO includes a parallel combination of an inductor and a capacitor along with a cross-coupled transistor pair.
FIG. 1 illustrates a resonant tank circuit 100 of the prior art. More particularly, the tank circuit 100 includes inductors L1 and L2, each having a first terminal coupled to a supply voltage VCC. A second terminal of the inductor L1 forms a node N1 which is coupled to a first terminal of a capacitor C1, to a collector of a transistor Q1 and to a base of a transistor Q2. A second terminal of the inductor L2 forms a node N2 which is coupled to a second terminal of the capacitor C1 and to a collector of the transistor Q2 and to a base of the transistor Q1. Emitters of the transistors Q1 and Q2 are coupled together and to a first terminal of a resistor Rt. A second terminal of the resistor Rt is coupled to a ground node. An output from the tank circuit 100 is taken across the capacitor C1 and supplied to a buffer circuit 102.
In operation, a voltage signal formed across the capacitor C1 is generally a sinusoid which oscillates at the resonant frequency of the tank circuit 100. When the node N1 is at a higher voltage level than the level of the node N2, the transistor Q2 has a higher bias voltage than the transistor Q1. Accordingly, nearly all of the current through the resistor Rt passes through the right side of the tank circuit 100 (through the inductor L2 and the transistor Q2). This tends to reinforce the voltage at the node N1 being higher than the voltage at the node N2. Accordingly, this results in positive feedback in the tank circuit 100.
Eventually, however, because there is little or no current passing through the inductor L1 and the transistor Q1, the voltage at the node N2 tends to rise relative to the level at the node N1. In response, the bias on the transistor Q1 increases while the bias on the transistor Q2 decreases. This reduces the current in the right side of the tank circuit 100 and increases the current in the left side (through the inductor L1 and the transistor Q1). Eventually, nearly all of the current through the resistor Rt passes through the left side which reinforces the voltage at the node N2 being higher than the voltage at the node N1, through positive feedback.
Because there is little or no current passing through the right side of the tank circuit 100, the voltage at the node N1 tends to rise relative to the level at the node N2. In response, the bias on the transistor Q2 increases while the bias on the transistor Q1 decreases. Accordingly, the above-described cycle repeats. In this manner, current is alternately steered through the right and left sides of the tank circuit 100, thereby forming a sinusoidal signal across the capacitor C1.
The conventional oscillator circuit illustrated in FIG. 1 has the drawback in that it has a gain peak which is relatively distant from the zero phase crossing, which can result in frequency drift during settling. The amplifier section of the oscillator formed by Q1, Q2 and Rt has a relatively high capacitive output susceptance (low output capacitive impedance) which reduces tuning range. In addition, the tank circuit 100 requires the buffer circuit 102 which can increase phase noise by loading the tank circuit 100.
Therefore, what is needed is an improved oscillator circuit which does not suffer from the drawbacks exhibited by the prior art oscillator circuit of FIG. 1.
In addition, so that a carrier signal used for transmitting signals in a wireless communication system is appropriately modulated or demodulated, it is important that the nominal center frequency of a VCO used in the system be equivalent to the center frequency of the frequency band covered. The center frequency, however, will generally be affected by process variations which affect the values of the various components of the VCO. Accordingly, the VCO must generally be tunable to compensate for these process variations.
Accordingly, what is needed is an improved technique for tuning a VCO.
An integrated VCO is a VCO which is formed as a portion of an integrated circuit, rather than with discrete components. Because the reactive elements of an integrated VCO are not as accessible as those of a VCO which is formed of discrete components, this creates a difficulty in tuning an integrated VCO.
Therefore, what is further needed is an improved technique for tuning an integrated VCO.
The invention is a fuse-trimmed tank circuit for an integrated voltage-controlled oscillator (VCO). An improved oscillator circuit in accordance with the present invention includes an L-C portion; a cross-coupled pair of transistors, whose collectors are coupled to the L-C portion; a capacitor coupled across the emitters of the cross-coupled pair of transistors; and a pair of resistors, where each resistor is coupled between a corresponding one of the emitters and a ground node.
The capacitor coupled across the emitters of the cross-coupled pair of transistors counteracts the effects of phase lag caused by the transistors. Accordingly, this oscillator circuit arrangement provides improved performance in that it results in a high small-signal loop gain which provides reliable start-up; has a peak gain which is closer to a zero phase crossing than prior tank circuits, which reduces frequency drift during settling; and has a low capacitive output susceptance. Also, a periodic output signal formed by the VCO can be taken at the emitters of the cross-coupled pair, which avoids having to utilize a buffer which could increase phase noise.
The present invention also provides a technique for trimming the VCO so as to adjust its center frequency. Capacitance for the L-C portion of the tank circuit is provided by one or more varactor diodes. When the VCO is operational, the varactors are under reverse bias conditions. Accordingly, the varactors act as capacitive elements. One or more of the varactors has an associated fuse coupled in series with the corresponding varactor. To trim the center frequency of the VCO, the capacitance of the L-C portion is adjusted by selectively blowing the fuses. This reduces the capacitance of the L-C portion by an amount equivalent to the capacitance of the varactors associated with the blown fuses.
Under normal operating conditions, currents through the fuses are sufficiently low so as to avoid blowing any of the fuses. To blow a fuse, however, a dc voltage is applied to forward bias the corresponding varactor such that the resulting current is sufficiently high to blow the fuse. In the preferred embodiment, the varactors are arranged in pairs, where each varactor of the pair contributes to the capacitance of one or the other sides of the tank circuit for maintaining its symmetrical and differential nature. Preferably, the cathodes of each pair of varactors are coupled together and to a corresponding switch. The dc voltage is selectively applied to the pair of diodes via the corresponding switch. Accordingly, the two fuses associated with each varactor of the pair are blown together.
In the preferred embodiment, the VCO is fully integrated into an integrated circuit. Accordingly, the present invention has the advantage of not requiring elements external to the integrated circuit for trimming the VCO.